Cord blood (CB) hematopoietic stem cell (HSC) transplantation plays a growing role in the treatment of a wide variety of malignant and non-malignant disorders such as leukemia, lymphoma, lymphoproliferative disorders and bone marrow failures (1, 2). Cord blood, as a source of HSCs, widens the pool of potential donors compared to bone marrow and peripheral blood stem cells due to its ease of harvest, availability, less stringent HLA matching criteria and lower graft-versus-host disease. However, despite the advantages, the number of CB HSC transplantations recorded in a 2008 survey in Europe is only 7% of the total allogeneic HSC transplantations (3). This is due to the low number of cells collected per unit of CB that restricts its use to children and lightweight recipients. This cell dose limitation leads to a lower success rate in adult recipients, marked by a delay in engraftment and vulnerability to infectious morbidity (2). In order to address cell dose limitations, differing strategies to expand CB HSCs ex vivo have been proposed. Importantly, ex vivo expansion is geared not only to increase the number of transplanted cells, but the number of lineage committed progenitor cells that can accelerate the engraftment process and reduce the risk of infection. Thus far, only one research group shows a marked improvement in the engraftment rate of ex vivo expanded CB HSC in a phase 1 clinical trial (4).
One of the attempts to improve HSC expansion ex vivo includes the incorporation of stromal components in culture to recreate the hematopoietic microenvironment in which stroma derived extracellular matrix (ECM) and stem cells provide complex molecular cues to support hematopoiesis (5-9). Up to 2 decades ago, it was believed that direct physical contact between HSC and stromal components was required for HSC maintenance (10, 11). However, more recent studies show that stromal cell-derived conditioned media, in which various cytokines and proteoglycans are found, is sufficient to maintain HSCs (7, 12).
Bone morphogenetic protein-2 (BMP-2) has been shown to be efficacious for the treatment of critical-sized bone defects with results comparable to autologous bone graft (35-37). As BMP-2 treatment replaces the need for autologous bone graft, its use is associated with shortened hospital stays for patients (38). However, the high dose of BMP-2 required for a successful therapy carries with it an increased risk of side effects and a greater economic burden for the healthcare system (38). To reduce the efficacious dosage, efforts have focused on improving BMP-2 half-life and/or sustaining and localizing its release (39-43). Heparin has been investigated extensively and shown great promise in this regard. Heparin, a hyper-sulfated glycosaminoglycan (GAG) sugar harvested from mast cell-rich tissues, can bind to and modulate various extracellular molecules including growth factors, adhesion molecules, and receptors (44). Heparin is known to bind to BMP-2 and slow its degradation (44, 45) and inhibition by noggin (45, 46), thereby enhancing its osteoblastic activity. It has also been suggested that heparin prevents BMP-2 from binding to endogenous cell-surface HSPGs, thus enhancing its bioavailability (43). The use of heparin in bone repair has been extended to various types of scaffolds as a means to increase the incorporation of BMP-2 and sustain its delivery in vivo, thereby improving BMP-2 efficacy (39, 41, 47-49).
Despite promising results, the use of heparin to augment BMP-2 therapy may pose unwanted effects due to heparin's affinity for a wide range of proteins. For instance, heparin's binding to antithrombin III is widely used for anticoagulant therapy (50). As the fracture hematoma acts as a reservoir for cytokines and growth factors important for bone repair (51), anticoagulant compounds like heparin may be considered counter-productive. Furthermore, heparin treatment is known to reduce bone density and has been linked to the development of osteoporosis (52, 53) through its pro-osteoclastic actions in vitro (54, 55) and in vivo (53). Indeed, heparin is known to inhibit the interaction between RANKL, a cytokine responsible for the formation and activation of osteoclasts, and its decoy receptor osteoprotegerin (56).
Bone morphogenetic protein 2 (also called bone morphogenic protein 2, BMP2 or BMP-2) is a member of the TGF-β superfamily strongly implicated in the development of bone and cartilage. It is an osteogenic protein, i.e. is a potent inducer of osteoblast differentiation (Marie et al. (2002) Regulation of human cranial osteoblast phenotype by FGF-2, FGFR-2 and BMP-2 signaling”. Histol. Histopathol. 17 (3): 877-85). Implantation of collagen sponges impregnated with BMP2 has been shown to induce new bone formation (Geiger M, Li R H, Friess W (November 2003). Collagen sponges for bone regeneration with rhBMP-2. Adv. Drug Deliv. Rev. 55 (12): 1613-29.). Recombinant human BMP2 is available for orthopaedic use in USA (e.g. INFUSE® Bone Graft, Medtronic Inc, USA).